In the process for manufacturing integrated circuits, SOI (silicon on insulator) substrate is usually used in conjunction with STI (shallow trench isolation) to achieve complete isolation between devices. FIGS. 4a-d show a method for manufacturing an SOI substrate in the prior art. In the method, silicon wafers are used to manufacture the SOI substrate, and two silicon wafers are needed for forming each SOI substrate. FIGS. 5a-d show another method for manufacturing an SOI substrate which improves the utilization efficiency of silicon wafer by using Smart Cut technology. FIG. 6 shows that STIs are formed in an SOI substrate in order to achieve complete isolation between devices. The methods for forming SOI substrate in the prior art are very complicated and expensive because a large number of silicon wafers are used, which significantly limits wide application of SOI substrates in IC industry. Therefore, a technique is needed to replace the methods for forming SOI substrate in the prior art so as to simplify process and reduce costs.